| Deep manned submersible in the complex ocean environment will be affected by the effect of external pressure,frequent dives and floatation process will cause the cycle changes of structural stress,in addition,other studies have suggested a manned submersible in great depth under constant depth work also will be affected by load over a long period of continuous load,the load will accelerate the function of pressure spherical shells of fatigue damage,and even cracks to ensure the safety of the scientific research personnel and inspection equipment.Due to the influence of many uncertain factors,the traditional fatigue analysis method is difficult to evaluate the damage accurately.The reliability theory can describe the distribution characteristics of uncertain variables and get more accurate evaluation.Therefore,it is very important to study the fatigue reliability of the material of the pressure shell of submersible.At present,with the manned submersible constantly challenging the limit of deep sea diving,the traditional pressure-resistant shell manufacturing material high-strength steel has been unable to meet its performance requirements.Titanium alloy has mature mechanical processing properties and excellent mechanical properties,and is an important metal material for manufacturing pressure-resistant structures.However,the crack propagation phenomenon will occur in titanium alloy under the influence of many uncertain factors.Therefore,the application of probabilistic reliability method in the field of fatigue analysis of titanium alloy is of great significance for fatigue design and reliability analysis of deep-sea manned submersible.In this paper,based on the theory of fracture mechanics,the improved Mc Evily and the fatigue model considering the load-bearing effect were combined with the reliability theory to establish the limit state calculation model respectively,and the fatigue reliability and failure probability under three different stress-load ratios were obtained.The fatigue reliability and load-fatigue reliability of a new titanium alloy for the pressure shell of deep-sea manned submersible have been studied.The main research contents of this paper are as follows:(1)The methods for evaluating the fatigue life of Marine structures are summarized,including the traditional theory of cumulative fatigue damage and the theory of crack propagation based on fracture mechanics.This paper introduces the reliability theory and its development process,and compares and analyzes two reliability methods,the first-order second-moment method and the response surface method.(2)Basic mechanical properties test,fatigue crack propagation rate test and load-fatigue crack propagation rate test of new titanium alloy materials of manned submersible were carried out respectively.The original data of the new titanium alloy materials were obtained,that is,the test data under different stress ratios,and the validity of the data was verified by analysis,which laid a foundation for the subsequent fatigue reliability analysis.(3)Based on the improved Mc Evily crack propagation model,the uncertain factors in the model were analyzed,and the data fitting of the uncertain parameters was carried out by using the normal distribution and weibull distribution function,and the statistical distribution characteristics of the main uncertain parameters were obtained.Based on the fracture failure criterion and the reliability theory,the limit state equation is established,and the reliability under different stress ratios is calculated by the first-order second moment method and the response surface method respectively.(4)Based on fatigue and load,on the basis of experimental study,considering the load of the fatigue crack propagation life of load,and limit state equation is established according to the fracture failure criterion,respectively,by using a second order moment method and response surface method for reliability computation,is obtained under different stress ratio of the reliability index and failure probability.The sensitivity analysis of uncertain parameters with stress ratio R of 0.3 was carried out,and the reliability of the parameters decreased with the increase of the coefficient of variation.Finally,the fatigue reliability under different crack propagation models is compared and analyzed,and the influence of the guaranteed load on the uncertainty and reliability is demonstrated.In this paper,the characteristic distribution of the uncertain parameters of the new material is fully considered by combining the fracture mechanics crack propagation theory with the reliability theory.The reliability of fatigue crack propagation in new titanium alloy was studied by means of statistical distribution and sensitivity analysis of uncertain parameters. |
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